Transcript Line effects RMWG

VARIABLE SPEED DRIVES
HARMONIC EFFECTS
&
EFFECT OF USE WITH LONG CABLES (100 – 300m)
Electric Motors & Drives
….OUR VISION, YOUR SOLUTION …….
TOGETHER WE DRIVE THE FUTURE!
HARMONIC EFFECTS
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VSD Typical Input current wave form
N.B. This effect is common and similar
to all Voltage Source PWM VSDs
T
1>
1>
Current
1) Ref A:
1 0 A 5 ms
Power
Supply
3-ph
Motor
3
Input current (Ie)
Input Current distortion resulting from a VSD
2000
1500
1000
0
-500
-1000
-1500
-2000
0
0.005
0.01
0.015
0.02
0.025
0.03
Voltage at PCC
time (s)
Input Voltage distortion resulting from a VSD
800
600
400
200
Voltage (Volts)
Current (Amp))
500
0
-200
-400
-600
-800
0
0.005
0.01
0.015
time (s)
0.02
0.025
0.03
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Vt   2.Vrms. sin . t 
1
V3H ( t )   . 2 .Vrms. sin( 3..t )
 3
1
V5 H ( t )   . 2 .Vrms.sin(5..t )
5
1
1
1


Vtot (t )  2 .Vrms . sin .t   .sin 3..t   .sin 5..t   .sin 7..t   ...
3
5
7


5
Voltage
Input sine wave
4%at PCC
reactor, 2.4% THD
1000
1000
800
800
600
600
400
400
200
200
Voltage (Volts)
Voltage (Volts)
at PCC
Input sineVoltage
wave
0% THD
0
-200
0
-200
-400
-400
-600
-600
-800
-800
-1000
-1000
0
0.005
0.01
0.015
0.02
0.025
0.03
0
0.005
0.01
time (s)
0.02
0.025
0.03
time (s)
Voltage
at PCC 8% THD
Input sine
wave
Voltage at PCC
Input sine wave
1% reactor, 3.6% THD
800
800
600
600
400
400
200
200
Voltage (Volts)
Voltage (Volts)
0.015
0
0
-200
-200
-400
-400
-600
-600
-800
-800
0
0.005
0.01
0.015
time (s)
0.02
0.025
0.03
0
0.005
0.01
0.015
time (s)
0.02
0.025
0.03
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HARMONICS – POSSIBLE SOLUTIONS
1. Use an AC line reactor
2. VSD design with built in DC choke
3. Install harmonic filters
4. Use 12 pulse VSD
5. Use regenerative / active front end VSD design
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HARMONICS – POSSIBLE SOLUTIONS
SUMMARY
VSD Type
Typical I THD
Cost
Standard 6 pulse
101%
100%
6 pulse with 2% reactor
56%
103%
6 pulse with 4% reactor
39%
105%
12 pulse
12%
200%
Regenerative
3%
250%
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HARMONICS –
THE WEG SOLUTION:
1. All 525V WEG VSDs >32A have a built in DC choke.
2. In all other cases, or for greater harmonic reduction use a line reactor
of 2 – 4% rating. In most installations this is an acceptable solution.
3. For high power ≥500kW applications consider a 12 pulse VSD or
active front end design if harmonics levels are a cause of concern.
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INPUT REACTORS –
WHAT IS IMPORTANT TO REMEMBER:
1. The harmonic values for all reputable AC VSDs are very similar.
2. The Voltage harmonic distortion value is affected by the customer
supply transformer rating and electrical network – We cannot give a
value without knowing his system detail
3. Built in AC reactors have little technical value – it’s mostly a
marketing exercise
4. Built in DC chokes have technical value
1. Give a similar result to a 2% AC line reactor
2. Do not cause a volt drop ( as an AC reactor does )
3. All the WEG 525V VSDs >32A have a built in DC choke
5. Input reactors are also useful to reduce the effect of dips, sags,
swells, transients and other line side disturbances
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MAIN PROBLEMS ASSOCIATED WITH LONG
CABLES BETWEEN VSD AND MOTOR
1. Capacitive leakage current
2. Voltage reflections causing high Vpeak and
dv/dt values
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CAPACITIVE LEAKAGE CURRENTS
VSD
T
1>
1>
Motor
1 ) Ref A :
5 A 5 ms
1. The VSD output is not a pure and balanced sinusoidal waveform
as supplied by Eskom
2. On long cables the cable conductor and insulator properties
combine to behave like a capacitor
3. The combination causes capacitive leakage current of significant
values in cables > 100m
4. This effect is common to all PWM VSDs
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CAPACITIVE LEAKAGE CURRENTS
THE SOLUTION
VSD
Motor
1. Compensation techniques within the VSD ( This avoids nuisance
VSD tripping, but does not necessarily eliminate the capacitive
leakage currents )
2. Additional reactance in the VSD output circuit to counter the
capacitance, i.e. an output reactor
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VOLTAGE REFLECTIONS CAUSING HIGH
Vpeak & dv/dt
VSD
Motor
Voltage pulses from the PWM on the VSD output are reflected
at the motor terminals causing voltage pulses significantly
higher than the motor nominal voltage
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VOLTAGE REFLECTIONS – THE REALITY
T
1>
1>
T
1 ) Ref A :
PWM theory
1>
1>
2 0 0 V o lt 2 ms
1 ) Ref A :
5 A 5 ms
Actual VSD output measurement
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Voltage reflection measurements
VSD Output
Motor terminals
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VOLTAGE REFLECTIONS –
Actual measurements
VSD Output
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VOLTAGE REFLECTIONS –
Actual measurements
Motor terminals
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Influence of the cable length
According to NEMA application guide:
• 1.5m cable
 overshoots beginning (overvoltages)
• 15m cable
 may reach 2 times VDC Link
• Over 120m
 peaks could be higher than 2VDC Link (longer time)
VSD + Standard Motor (7.5kW-380V- IV p)
VSD terminals
without motor
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1.5 m cable
34 m cable
15 m cable
54 m cable
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VOLTAGE REFLECTIONS –
Actual measurements
380V supply, 65 m armoured cable, 5.5kW motor & VSD
No reactor
Vp = 975V
±2.6 x Vsupply
5% reactor
Vp = 667V
±1.8 x Vsupply
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SUMMARY – VOLTAGE REFLECTIONS AND DV/DT AT MOTOR TERMINALS
1400V
50m+ cable
1200V
1000V
50m cable +choke
5m cable
800V
VSD output
600V
525V
400V
Normal 525V sine wave
200V
10mSec.
5mSec.
0V
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VOLTAGE REFLECTIONS & DV/DT –
THE WEG SOLUTION:
1. Design the VSD so that the VSD output dv/dt is low enough to
minimise the Vpeak and dv/dt at the motor terminals
2. Design the motor insulation to withstand Vpeak and dv/dt values in
excess of that which the VSD will cause –
1. Generation 2 wire – Vp ≤ 1430V & dv/dt ≤ 5200 V/µsec.
2. Generation 3 wire – Vp ≤ 1780V & dv/dt ≤ 6500 V/µsec.
3. Output chokes are always advisable and beneficial to use, but not
always mandatory
1. Counteract capacitive leakage currents
2. Reduce Vpeak
3. Reduce dv/dt
4. Simple and cost effective to use
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END
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